1. Field of the Invention
The present invention relates to a driving device composed of an ultra-small motor, and particularly to an aperture blade driving device using the same.
2. Related Background Art
FIG. 18 shows one of small-sized cylindrical step motor in the prior art. A stator coil 105 is wound around a bobbin 101 concentrically. The bobbin 101 is fixed by being axially held and fixed by two stator yokes 106 and 106. Each stator yoke 106 has stator teeth 106a and 106b that are alternately placed on the inner circumferential surface and in the circumferential direction of the bobbin 101. The stator yoke 106 integrated with the stator teeth 106a or 106b is fixed to case 103 thereby comprising a stator 102.
Of two cases 103 and 103, one case 103 has a flange 115 and a bearing 108 that are fixed thereto whereas the other case 103 has another bearing 108 fixed thereto. A rotor 109 is composed of a rotor magnet 111 fixed to a rotor shaft 110. A radial void is formed between the rotor magnet 111 and the stator teeth 106a of stator 102. The rotor shaft 110 is rotatably supported between the two bearings 108 and 108.
FIG. 20 shows a step motor that is one of those driven by a single coil and is frequently used in watches. Denoted by 201 is a rotor composed of a permanent magnet, and 202 and 203 denotes stators while 204 denotes a coil.
The conventional small-sized step motor shown in FIG. 18 has a drawback that the external dimension of the motor is large because the case 103, the bobbin 101, the stator coil 105 and the stator yoke 106 are arranged concentrically about the outer periphery of the rotor. This step motor has another drawback of low output, for, as shown in FIG. 19, the flux of magnetic force generated upon energization of the stator coil 105 mainly passes between an end face 106a1 of the stator tooth 106a and an end face 106b1 of the stator tooth 106b and does not act on the rotor magnet 111 effectively.
The step motor shown in FIG. 20 is also ineffective in making the flux of magnetic force that is generated upon energization of the coil act on the magnet because the flux of magnetic force concentrates on a small gap between the stator 202 and the stator 203. The inventors of the present invention have proposed a motor to solve these problems in Japanese Patent Application Laid-Open No. 9-331666 (U.S. Pat. No. 5,831,356).
The motor proposed has a rotor formed into a cylinder and composed of a permanent magnet that is divided into equal parts along the circumferential direction and is magnetized so as to form different poles alternately. In the axial direction of the rotor, a first coil, the rotor and a second coil are arranged in this order. The first coil excites a first outer magnetic pole that faces the outer peripheral surface of the rotor as well as a first inner magnetic pole that faces the inner peripheral surface of the rotor. The second coil excites a second outer magnetic pole that faces the outer peripheral surface of the rotor as well as a second inner magnetic pole that faces the inner peripheral surface of the rotor. The rotation axis that is the rotor shaft is taken out the interior of the cylindrical permanent magnet.
The motor structured as above has a high output and a small motor external dimension, but there is room to improve it in facilitating joining of the rotor shaft to the permanent magnet. If a thin magnet is used in the structure above, the distance between the first outer magnetic pole and the first inner magnetic pole as well as the distance between the second outer magnetic pole and the second inner magnetic pole can be closed, resulting in a reduction in magnetic resistance in the magnetic circuit. This makes it possible to generate more flux of magnetic force with a small amount of current flowing through the first coil and the second coil.
However, the motor disclosed in Japanese Patent Application Laid-Open No. 9-331666 and other motors of that type are required to be long to a certain degree in the axial direction. For that reason, there has lately been a demand for a drive transmission device composed of a ultra-small motor whose axial length is very short. The drive transmission device composed of a ultra-small motor whose axial length is very short is particularly needed as a drive transmission device used in a lens barrel of a camera in order to drive an aperture blade, a shutter, a lens, etc.